To win the presidency, a candidate must win a majority of the votes in the Electoral College. The Electoral College has electors equal to the number of members in Congress (535 representatives in the House and 100 senators) plus three electors for the District of Columbia (which were granted in 1961 by Amendment XXIII to the U.S. Constitution).

With the exception of Maine and Nebraska, states award all their electors to the candidate that wins a majority of the popular vote in that state.

A candidate, therefore, can win the election without winning the popular vote if they barely win the popular vote in the states they carry and lose by large margins in the states they do not carry.

To illustrate, let us assume that the United States is composed of three states, each with a population of 100 and House districts each have 100 people. Each state would have three electors (two Senators plus one House member) and five electors would be needed to win.

Suppose also that there are two candidates. Candidate one wins two states with 51 votes (turnout is 100 percent), but loses the third state where they got only one vote. This means that candidate two got 49 votes each in the first two states and 99 votes in the third state. Who wins?

Candidate one wins six electors for winning the popular vote in two states even though they only received 103 votes (51 + 51 + 1). Candidate two wins three electors for winning one state but received 197 votes (49 + 49 + 99).

Candidate one, therefore, would win the presidency even though they received nearly half as many votes as candidate two.

Though an extreme example, the hypothetical illustrates how a candidate can win the presidency without winning the popular vote. If this were to happen next Tuesday, it would mean that the winner barely won the popular vote in a few states, but lost the popular vote by larger margins in some other states.